Method for negotiating weakened keys in encryption systems

Abstract

A method for permitting encrypted communications between two stations which are operable with encryption algorithms that accept encryption keys having work factors with different values, by: in a first determining step, determining the lower one of the different values; providing an initial encryption key having a first work factor value; comparing the first work factor value with the lower one of the work factors determined in the determining step; when, in the comparing step, the first work factor value is greater than the lower one of the work factor values determined in the determining step, performing the following steps: performing a first hash function on the initial encryption key to produce a first output, and deriving from the first output a first intermediate key having a work factor value not greater than the lower one of the different work factor values determined in the determining step; performing the first hash function on the first intermediate key to produce a second output, and deriving from the second output a final encryption key having a work factor value not greater than the lower one of the different work factor values determined in the determining step; and using the final encryption key to encrypt communications between the two stations; and when, in the comparing step, the first work factor value is found to not be greater than the lower one of the work factor values determined in the determining step, using the initial encryption key to encrypt communications between the two stations.

What is claimed is:  
     
       1. A method for permitting encrypted communications between two stations which are operable with compatible encryption algorithms that accept encryption keys having work factors with respectively different values, comprising: 
       in a first determining step, determining the lower one of the different work factor values;  
       providing an initial encryption key having a first work factor value;  
       comparing the first work factor value with the lower one of the work factor values determined in said determining step;  
       when, in said comparing step, the first work factor value is found to be greater than the lower one of the work factor values determined in said determining step, performing the following steps:  
       performing a hash function on a first word that includes the initial encryption key to produce an intermediate key, and deriving from the intermediate key a modified intermediate key having a work factor value not greater than the lower one of the different work factor values determined in said determining step;  
       performing a hash function on a second word that includes the modified intermediate key to produce a second output, and deriving from the second output a final encryption key having a work factor value not greater than the lower one of the different work factor values determined in said determining step; and  
       using the final encryption key to encrypt communications between the two stations; and  
       when, in said comparing step, the first work factor value is found to not be greater than the lower one of the work factor values determined in said determining step, using the initial encryption key to encrypt communications between the two stations.  
     
     
       2. The method of  claim 1  further comprising providing a multi-bit data item constituting a salt, and wherein at least one of the first and second words is a combination of the respective key and the salt. 
     
     
       3. The method of  claim 2  wherein each of the first and second words is the combination of the respective key and the salt. 
     
     
       4. The method of  claim 3  wherein each of the first and second words is a concantenation of the respective key and the salt. 
     
     
       5. The method of  claim 1  wherein the intermediate key has a bit length determined by the hash function and a work factor value equal to the lower one of the different work factors. 
     
     
       6. The method of  claim 1  wherein the hash function performed on the first word is identical to the hash function performed on the second word. 
     
     
       7. A method for permitting encrypted communications between two stations, each of which stations is operable with an encryption algorithm that can accept an encryption key having a given work factor value, comprising: 
       providing an initial encryption key having a first work factor value which is smaller than the given work factor value of the encryption key that can be accepted by each station;  
       performing a hash function on a first word that includes the initial encryption key to produce an intermediate key, and deriving from the intermediate key a modified intermediate key having a work factor value which has a greater resistance to precomputation attack than the first work factor value and which is not greater than the given work factor value of the encryption key that can be accepted by each station;  
       performing a hash function on a second word that includes the modified intermediate key to produce a second output, and deriving from the second output a final encryption key having a work factor value equal to the work factor value of the first intermediate key; and  
       using the final encryption key to encrypt communications between the two stations.  
     
     
       8. The method of  claim 7  further comprising providing a multi-bit data item constituting a salt, and wherein at least one of the first and second words is a combination of the respective key and the salt. 
     
     
       9. The method of  claim 8  wherein each of the first and second words is the combination of the respective key and the salt. 
     
     
       10. The method of  claim 9  wherein each of the first and second words is a concantenation of the respective key and the salt. 
     
     
       11. The method of  claim 7  wherein the intermediate key has a bit length determined by the hash function performed on the first word and a work factor value equal to the lower one of the different work factors. 
     
     
       12. The method of  claim 7  wherein the hash function performed on the first word is identical to the hash function performed on the second word.